The invention relates generally to circuit breakers and, in particular, to circuit breakers having an improved calibration screw. The invention further relates to an improved method for calibrating a circuit breaker bi-metal to a permanent setting.
Molded case circuit breakers provide overcurrent protection for residential, and some commercial and industrial electrical circuits. These circuit breakers are generally installed in lighting or distribution load centers to supply electrical load at lower voltages and currents. A 20 Amp circuit that supplies 120 V electrical outlets in a residence is one example. The molded case circuit breaker is typically installed in a distribution load center with other like breakers. Lighting circuit installations are also characterized by the installation of multiple molded case breakers in a single load center. A load center consists of a sheet metal enclosure with a hinged door that allows access to the face of the enclosed molded case circuit breakers. The circuit breakers are secured within an inner sheet metal panel. Electrical busses and conductor raceways are located beneath this inner panel. The molded case breakers generally include a molded case main breaker supplying at least one common bus located within the load center. Multiple molded case circuit breakers are then used to distribute power to external electrical load. These xe2x80x9cdistribution breakersxe2x80x9d are connected to both the common bus and external circuits that supply the electrical load. The distribution breaker line stab is connected to the common bus and the external electrical circuit is connected to a circuit breaker terminal lug. Generally, each distribution breaker supplies a single electrical circuit that may supply multiple remote electrical loads.
Molded case breakers are generally inexpensive and non-serviceable pieces of equipment. Therefore, manufacturers have attempted to design these circuit breakers for low cost assembly. U.S. Pat. No. 3,464,040 entitled xe2x80x9cCompact Circuit Breaker Constructionxe2x80x9d, herein incorporated by reference, discloses a one-half inch residential molded case circuit breaker designed for economical fabrication on mass production equipment. Manufacturers next turned to robotic assembly. U.S. Pat. No. 4,513,268 entitled xe2x80x9cAutomated Q-Line Circuit Breakerxe2x80x9d, herein incorporated by reference, discloses a molded case circuit breaker designed for completely automated assembly and calibration.
Molded case breakers usually include a thermal element in the form of a bi-metal that initiates a circuit breaker trip for low overcurrent conditions. Most molded case also include a magnet and armature that combine to initiate a circuit breaker trip for higher magnitude overcurrents. The assembly taught in the aforementioned U.S. Pat. No. 4,513,268 includes a calibration screw assembly used to adjust the bi-metal and calibrate the overcurrent protection. The calibration screw assembly uses a calibration screw with two opposing ends. A first end having a head and a second end having a tip. The head and the tip are each designed to perform a single specialized function that cannot be performed by the other. Therefore, the shape of the head differs from the shape of the tip. The head is shaped to allow a screwdriver to cooperatively engage the head and apply rotational force to drive the screw in or out of a threaded receptacle. The opposing tip has a flat tip that engages the circuit breaker bi-metal. The bi-metal deflects as the calibration screw is screwed into a threaded receptacle, adjusting the pivot point of the bi-metal as the screw penetration is increased. The tripping current level is adjusted when the bi-metal deflects. The circuit breaker calibration is fixed when sealant or epoxy is applied to the head of the screw to lock it in place after the desired calibration is established.
However, the flat longitudinal shape of the tip creates a constantly shifting point of contact between the bi-metal and the calibration screw as the screw is adjusted. This creates an uneven calibration adjustment whereby the amount of bi-metal 36 adjustment is not consistent throughout the full 360xc2x0 of screw rotation. Additionally, the calibration screw taught in U.S. Pat. No. 4,513,268 can only be installed in a single direction because the nut cannot receive the head of the screw and a screwdriver cannot be effectively applied to the tip. This unidirectional screw is particularly troublesome where automated assembly or other high speed manufacturing is used because proper screw orientation is limited to single position. The method of sealing the screw in place is also not optimal. The sealant will only reach the screw head and the threads nearest the head that it is forced into. This creates a seal that can be more easily broken due to shock or vibration when compared with improved methods. The breaker calibration setting may also unintentionally shift at this time.
Thus, there is a particular need for a calibration screw that provides a consistent calibration adjustment and is not limited to a single orientation. A calibration screw with improved features would reduce manufacturing defects, increase manufacturing speed and improve the calibration adjustment of molded case breakers. Further advantages could be gained by an improved method of sealing the calibration screw in place following completion of the calibration.
According to the present invention, the foregoing and other objects and advantages are attained by a calibration screw that includes a cylindrical body, a first end and a second end. The first end and the second end having common features including a convex tip surface. The cylindrical body is shaped to allow sealant to flow below the screw head when the calibration screw is in installed in a circuit breaker. A calibration method that permanently seals the calibration screw in place following adjustment is also included in the present invention.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.